1. Field of the Invention
The present invention relates to a surface protective film used for transparent conductive films and transparent conductive films with a surface protective film that are widely used in a field of a transparent electrode and the like, such as liquid crystal displays, touch panels, sensors, and solar cells.
2. Description of the Related Art
In general, thin films having transparency in visible radiation region and conductivity are used in transparent electrodes in new displays for liquid crystal displays, electroluminescence displays, etc., in touch panels, etc. and further in electrification prevention and electromagnetic wave interception, etc. for transparent goods. For example, in manufacturing process for touch panels by electric conductive film contact type where transparent electrodes of transparent substrates having transparent electrode with transparent conductivity are arranged facing each other on one side, various kinds of complicated and long treating processes and assembling processes, such as cutting of the transparent conductive film, printing of resists, etching, printing of silver ink, and further delivery after arranging transparent electrodes of two sheets of conductive films facing each other are usually carried out.
In manufacturing processes of touch panels, surface protective films are conventionally used in order to prevent occurrence of soil, and scratch, and others in transparent conductive films. As mentioned above, however, since touch panels have complicated and long manufacturing process, different kinds of surface protective films are newly replaced and used for each manufacturing process, in other words, different surface protective films are properly used for different manufacturing processes. Accordingly, manufacturing process becomes complicated very much and deterioration in working efficiency and in yield raises manufacture cost.
Therefore, unification of variety of surface protective films used in a manufacturing process of touch panels has been strongly desired. As characteristics required for unification of variety of the films, there may be mentioned heat resistance, which does not melt a base material of the surface protective film in a heating process at about 150° C. required for drying process for silver ink printing during touch panel manufacturing processes. Since conventional surface protective films of polyolefin based materials, such as low density polyethylenes and polypropylenes, gives problems of melting or large deformation, they cannot be used for the above-mentioned heating processes. On the other hand, in use of films having outstanding heat resistance, such as polyethylene terephthalate resin films, which does not melt in heating process at about 150° C., new problem occurs. That is, a difference in a rate of thermal shrinkage by heating between a transparent conductive film and a surface protective film curls the transparent conductive film with the surface protective film attached thereon (here, curling represents a phenomenon of deformation into concave shape with upper surface protective film facing inside). This curling phenomenon deteriorates workability on manufacturing process. And furthermore, when a direction of the curling faces a side opposite to the transparent electric conductive film, a large problem occurs in quality of touch panel that deformation by the curling will contact both electrodes of upper electrodes and lower electrodes in the transparent electric conductive film.